In the aluminum metallurgy, smelting and casting processes, there exist unavoidably harmful impurities in aluminum and the alloys thereof. On one hand, these impurities cause discontinuity in the metallographic structure, form the crack sources inside the structural parts, decrease the strength, plasticity and impact properties of the material; on the other hand they may also become the origin of chemical or electrochemical corrosion. In addition, the impurities have a strong adsorption of hydrogen, which is a leading culprit for the pinholes and porosity in aluminum castings. The generation of the oxidative impurities in aluminum is due to the physical or chemical changes that occurs on the interface between the aluminum melt and the ambient, or due to the gas entrapped by the turbulent flow during the casting and transfer of molten aluminum, etc. The methods for removing impurities in aluminum and the alloys thereof include floatation, fluxing and filtration, etc. The principle of removing impurities is to use various adsorptive mediums that have an adsorption effect on the impurities, such as inert or active gases, liquid flux, chloride salts or a filtration medium. In the mean time, a sufficient contact of the melt with the adsorptive medium ensures a physical, chemical or mechanical action, which results in the transfer of impurities from the aluminum melt to the adsorptive medium, hence the purified aluminum melt. To remove impurities with a flux, the most common method comprises spreading the flux onto the surface of an aluminum melt to adsorb the impurities in the molten aluminum; or employing a stirring operation to enhance the contact between flux and aluminum melt. In such methods, the processing time is longer, the impurity removing effect is not satisfied; and meanwhile air is easily entrapped during the stirring operation and secondary oxidation impurities are generated. In order to improve the impurity removing effect with a flux, some methods and purifying devices have been exploited. The relevant documents are listed as follows.
Flux Practice in Aluminum Melting, AFS Transactions, 1992, Vol. 88, pp. 737-742. This document discloses a flux injection method. In order to overcome the disadvantage of the conventional practices for limited contact with unwanted impurities in the aluminum melt. Flux injection overcomes this limitation by delivering predetermined amounts of powdered flux beneath the melt surface. Upon leaving the lance, the flux melts into small droplets that offer a large specific surface area with the melt as they float to the surface. This accelerates flux-induced metal cleaning.
Chinese patent publication CN98205426.2, A Graphite Purifier for Removing Impurities in Aluminum Melt Liquid. The structure of the purifier comprising: a purifier rotator, which is of gear wheel type; a purifier rotator shaft, of which one end is fixed on the purifier rotator; a purifier external connection chuck, of which the bottom is joined together with the upper portion of the purifier rotator shaft, and the top is connected to an external rotation driver mechanism; a vent hole, which axially goes through the purifier rotator, the purifier rotator shaft and the purifier external connection chuck, is characterized in that comprising, on the outside of the upper-to-middle part of the rotator shaft, a jacket layer of composite tubular type, which is tightly fixed on the external face of the rotator shaft; an reinforcement mantle layer of graphite tubular type, which is tightly fixed on the external face of the jacket layer of composite tubular type.
Chinese patent publication CN01139250.9, Device for eliminating non-metallic impurity in aluminum melt by Filtration. The device mainly comprises: a resistance furnace, a crucible, an agitator, a heat insulating cover, a steel barrel and a height adjustable lifter. The steel barrel is jacked externally the crucible, then they are disposed in the resistance furnace and fixed with a refractory material. The heat insulating cover and the resistance furnace are connected via a screw. The height adjustable lifter is inserted through an insert port in the heat insulating cover. The resistance furnace mainly comprises: a heating element and a heat insulating furnace shell. The heating element is provided inside of the hearth of the resistance furnace. The space between the hearth of the resistance furnace and the heat insulating furnace mantle is filled with ceramic cotton. The working principle is as follows: the flux and the aluminum ingot are placed in two crucibles respectively and a covering agent is placed in the crucible containing the aluminum ingot. Secondly, the power supply of the heating furnace is turned on. After both of the flux and the aluminum ingot are melted, the agitator is put into the melted flux for stirring, and then the aluminum melt is ladled with a spoon and poured into a flow passage in batches so as to enter the rotating melted flux. Lastly, the agitator is removed after the transfer of the aluminum melt has completed. Particularly, when the device is running, the process is carried out as follows: firstly, an active flux and an aluminum ingot are placed in two graphite crucibles inside of the furnace respectively. It is still necessary to place a covering flux (of which the ingredients are same with those of the active flux used for filtration) in the crucible containing the aluminum ingot. After both of the flux and the aluminum ingot are melted, the agitator is placed in the graphite crucible containing the flux. Then the aluminum melt is poured into the rotating flux. During the aluminum melt being agitated and filtered, the liquid level of the flux will rise with the addition of the aluminum melt. Therefore, there is a supporter that adjusts the height of the agitator so that the impeller of the agitator is always located in the flux layer. After all of the aluminum melts are transferred into the crucible containing the flux, an active agent is placed in the graphite crucible out which the aluminum melt is transferred. After the flux is melted, the agitator is placed into the graphite crucible containing the flux via the agitator inlet. Thereafter, the aluminum melt is poured into the rotating flux again. Each of the filtrations is to repeat the above-mentioned operations. By means of implementing this process repeatedly, it is possible to distribute the impurities in the aluminum melt continuously onto the surfaces of the aluminum droplets. At the same time, the aluminum droplets will also redistribute the impurities in the aluminum droplets in the rotating flux, so that the impurities in the aluminum droplets also have an opportunity to be distributed onto the surfaces of the aluminum droplets. Thus, the impurities on the surfaces of the aluminum droplets can pass through the aluminum film-flux interface and enter into the flux layer. The aluminum melt is purified with the flux, and when the times of filtration reach 4, the efficiency for removing impurities reaches 84%, the impurities more than 7 micrometers can be removed efficiently. Therefore, this melt filtration by agitating the flux improves dynamically the impurity removal effect with a flux.
Chinese patent publication CN200680004257.8, Non-sodium-based Flux and Process for Treating Aluminum Alloys by Using the Same. The patent application provides a non-sodium-based flux, which ensures a highly deslagging effect by preventing the adhesion and sedimentation of the unreacted flux when the flux is injected into a rotary degassing device, as well as a non-sodium-based flux for treating molten aluminum alloys and a process for treating aluminum alloys by using it. The process comprises: maintaining the state of the impeller of the rotator submerged in the above-mentioned molten aluminum alloy; spraying an inert gas and the flux to the molten metal from the above nozzle, and rotating the rotator at a speed of 200-450 rpm, so that the impurities or the like in the molten metal float upwards to the surface of the molten metal together with the fine bubbles and the flux, thus the degassing and deslagging are achieved. However, either in the flux injection method or in the rotator-assistant flux injection method, the equipment is complicated. In addition, the impeller is submerged in the aluminum melt for long time and rubs against the aluminum melt, which often results in the abrasion and spalling of the material.